X-ray sources can be operated with very large voltage differentials, such as for example from 10 kilovolts to 80 kilovolts (kV). Problems associated with the high voltages in x-ray sources include (1) a breakdown of insulative potting material, which surrounds an x-ray tube and electrically isolates it from other x-ray source components, and (2) instability caused by surface charges along an x-ray tube cylinder.
Illustrated in FIG. 8 is a longitudinal cross-sectional side view of an x-ray source 800 comprising an evacuated enclosure 101, a cathode 102 attached to the evacuated enclosure 101 and configured to emit electrons 104 within the enclosure, and an anode 103 attached to the evacuated enclosure 101, configured to receive electrons 104 emitted from the cathode, and configured to emit x-rays 108 in response to impinging electrons 104.
The cathode 102 can be configured to emit electrons by an electron emitter 111, such as a filament. The filament can be heated, such as by alternating current from an alternating current source 105. A large bias voltage differential may be created between the cathode 102 and electron emitter 111 and the anode 103 by a high voltage generator 109. The electron emitter 111 can be maintained at a very low voltage, such as for example −40 kV, and the anode can be maintained at ground 107 voltage. Due to the large voltage differential between the electron emitter 111 and the anode 103, and a high electron emitter 111 temperature, electrons can leave the electron emitter and be propelled towards the anode 103. X-rays 108 can be generated at the anode 103 in response to impinging electrons.
An x-ray source shell or casing (not shown) can also be maintained at ground 107 voltage. An electrically insulative potting material 106 can be used to isolate the large negative voltage of the cathode 102 and the evacuated enclosure 101 from the shell or casing.
Illustrated in FIG. 9 is a lateral cross-sectional side view of an x-ray tube 900 that is orthogonal to the longitudinal cross-sectional side view of the x-ray source of FIG. 8, taken along line 9-9 in FIG. 8. Illustrated in FIG. 10 is a chart 1000 showing a change in voltage from a voltage of the cathode Vc to a voltage of zero at an outer perimeter of the potting 201. Note that there is a sudden and large change in voltage at a transition 1002 from the cathode 102 to the potting 106. This sudden and large change in voltage also occurs at a transition from the evacuated enclosure 101 to the potting 106, especially in portions of the evacuated enclosure 101 closer or adjacent to the cathode 102.
This sudden and large change in voltage, or large voltage gradient at and near this transition point 1002 can result in problems such as a breakdown of the potting material 106 at this point and also a buildup of surface charges on a surface of the evacuated enclosure 101. The breakdown of the potting material 106 can result in a short circuit of the x-ray source from the evacuated enclosure 101 or cathode 102 to other components or the shell or casing. A buildup of surface charges can cause x-ray source instability. Thus it can be desirable to reduce this voltage gradient.